The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The-cost and cabling required discourages extending a four-wire circuit to a subscriber's premise from the local exchange. For this reason, the four-wire trunk circuits are coupled to two-wire local circuits, using a device called a hybrid.
Hybrid echo, the primary source of echo generated from the public-switched telephone network (PSTN) is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points.
Unfortunately, the hybrid is by nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the speech signal. Provided that the total round-trip delay occurs within just a few milliseconds, the echo generates a sense that the call is live by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
The effective removal of hybrid echo is one key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing solutions that can reduce echo from hybrid. By employing this technology, the overall speech quality is improved significantly.
It is known in the art to employ adaptive filtering to address hybrid echo cancellation. In an adaptive filter, the filter coefficients are based, in part, on feedback of filter output. Normalized Least Mean Square (NLMS) adaptive filtering is one method, popular in echo cancellation, to address reflections in the telephony system.
Where an excitation is highly correlated, e.g. a continuous tone, the NLMS algorithm performs poorly. Where NLMS adaptive filtering is employed, tone signals, such as those employed in telephony tone dialing systems, may have a detrimental effect on other components, such as noise level calculators, Echo Return Loss (ERL) calculator, and Echo Return Loss Enhancement (ERLE) calculator, etc.
U.S. Pat. No. 4,363,100 discloses a tone detector in the form of a digital correlator, in which PCM signal samples are delayed in a RAM for the correlation period. Although U.S. Pat. No. 4,363,100 addresses tone detection which could be used in an echo cancelling system, it does not use the infrastructure of echo cancelling system to detect the tones.
U.S. Pat. No. 4,669,114 discloses a digital progress tone detector, for connection to a telephone line and a PABX. Again, this disclosure could be used in an echo cancelling system but it does not use the infrastructure of echo cancelling system to detect the tones.
What is needed is a method of detecting tones in a NLMS adaptive filter echo canceling system, and alerting this system to the presence of such tones.